This invention relates to glass compositions which are capable of being ion exchange strengthened at temperatures below the strain point, the use of such glasses in chemical strengthening processes, and the chemically strengthened glass articles produced thereby. In particular, the invention deals with chemically strengthenable glasses containing lithium in which strengthening is carried out by replacing lithium ions in the glass with larger alkali metal ions, such as sodium, from an external source, typically a molten inorganic salt bath.
The basis for below-the-strain-point ion exchange strengthening techniques is U.S. Pat. No. 3,218,220 to Weber. Subsequently, many attempts have been made to produce glass compositions with enhanced ion exchange properties, chief among which are the Al.sub.2 O.sub.3 and/or ZrO.sub.2 containing glasses disclosed by Mochel in U.S. Pat. Nos. 3,485,702, 3,752,729, and 3,790,430. Such modified glass compositions yield greatly enhanced rates of ion exchange, but usually have relatively high melting and forming temperatures.
A lithium-alumino-silicate glass composition of the same general type as the present invention, and in which the melting and forming temperatures are advantageously similar to that of conventional soda-lime-silica flat glass, is disclosed in U.S. Pat. No. 3,615,320 to Junge et al. Despite the advantageous combination of properties of the glass compositions disclosed in said U.S. Pat. No. 3,615,320, its compositions suffer a serious drawback in their reliance on a substantial inclusion of B.sub.2 O.sub.3. Because of its volatility and corrosiveness to refractory furnace materials, the B.sub.2 O.sub.3 content of such glasses has been found to render melting difficult in conventional flat glass melting furnaces. Thus, it would be highly desirable if a glass were available exhibiting the desirable combination of properties possessed by the glasses of U.S. Pat. No. 3,615,320 but without the troublesome B.sub.2 O.sub.3 content.
In addition to relatively low melting and forming temperatures, rapid ion exchange rates, and chemical durability, it is also desirable for a glass composition of this type to be free from devitrification problems. Thus, the glass should have a working range (the difference between the forming temperature and the liquidus temperature) of at least 50.degree. F. Furthermore, undesirable surface devitrification of glass sheets made in accordance with the aforementioned U.S. Pat. No. 3,615,320 has been observed when the glass sheets are heated to temperatures required for bending the glass sheets to sharp bends. The resultant reduction in transparency renders the sharply bent sheets unsuitable for some intended uses such as aircraft glazing. Thus, it would also be desirable to eliminate such a tendency of the glass to devitrify at the surface when bending.
U.S. Pat. Nos. 3,357,876 to Rinehart, 3,433,611 to Saunders et al., and 3,529,946 to Fischer et al. may also be of interest to the present invention for their disclosure of ion exchange glasses containing lithium, alumina, and zirconia. The first two, however, require the inclusion of P.sub.2 O.sub.5, and ingredient which, like B.sub.2 O.sub.3, is volatile. The last mentioned patent discloses only broad, general ranges without suggesting the specific composition of the present invention.